This invention relates to machines in which one member is positioned relative to another. For example, it relates to coordinate measuring machines, machine tools, scanning machines and robots, in which a tool or probe is positioned relative to a workpiece.
More particularly, the invention relates to performing measurements or calibration on such machines.
In some aspects, the present invention is a development of the apparatus and methods described in our International Patent Application No. PCT/GB94/02593, published as WO95/14905, to which reference should be made. Reference is also directed to the apparatus and methods described in International Patent Applications Nos. WO91/03145 (Kearney and Trecker) and WO92/17313 (Geodetic Machines), and in European Patent Application No. 534585 (Ingersoll).
All the above applications show machines in which a tool, probe or other operator is mounted on one structure, for movement relative to another structure upon which a workpiece may be mounted. The tool, probe or other operator may be movable and the workpiece fixed, or vice versa. The relative movement is effected by six rams acting between the two structures. These rams can be controlled so as to produce any desired relative movement between the structure, with six degrees of freedom (three translational and three rotational). It is necessary to measure the movements of the rams, and one aspect of the present invention is concerned with such measurement.
A problem with the apparatus described in the various above-referenced patent applications is that of calibrating the movement of the machine. To achieve this, it is necessary to measure the relative position and orientation of the moving structure relative to the fixed structure, independently of the measurements of the extensions of the rams which produced that position and orientation. Another aspect of the present invention allows such calibration, both on the type of machine described in the above patent applications, and on more conventional machines.
According to one aspect of the present invention a method of calibrating a machine comprises the steps of attaching a first structure to one part of a machine, said structure having at least one spherical or part-spherical support thereon,
attaching a second structure to a second part of a machine, said structure having three spherical, or part-spherical supports thereon arranged in a triangular pattern,
interconnecting the two structures by at least three measuring bars each of which is attached to one of the supports on each structure,
producing relative movement between the two parts of the machine thereby causing relative movement between the two structures, determining the changes in the lengths of the measuring bars caused by the relative movement of the two structures, and
determining from such length changes the actual movements of the two structures.
According to one embodiment of the invention the first structure has three spherical or part-spherical supports thereon which are arranged in a triangular pattern, six measuring bars are provided, opposite ends of each bar being connected to one support on each structure. In such an embodiment, each support on both structures has the ends of two measuring bars connected to it.
According to another embodiment of the invention, the first structure comprises a single support disposed on the machine at the position which would normally be occupied by the tip of a tool or a measuring probe, three measuring bars are provided, opposite ends of each bar being connected to the single support on the first structure, and to one of the supports on the second structure.
According to another aspect of the invention there is provided apparatus comprising a first structure having at least one spherical or part-spherical support thereon, a second structure which has three spherical or part-spherical supports spaced in a triangular pattern thereon, and at least three members interconnecting the two structures.
The apparatus may be connected to a machine as a calibration device in which case the members may be passive measuring bars, or alternatively the members may be powered extensible struts capable of manipulating one of the structures relative to the other.
In one embodiment of the invention both of the structures have three spherical or part-spherical supports spaced in a triangular pattern thereon, and six members are provided, each of which is connected at one of its ends to a support on one of the structures and at the other of its ends to a support on the other one of the structures. In such an embodiment each support on both structures has the ends of two members connected to it.
The powered struts may also incorporate measuring devices so that they additionally act as measuring bars for determining the relative movements of the two structures.
In any one of the above-described embodiments of any of the aspects of the invention, the supports may take the form of spheres or part-spheres, or of spherical or part-spherical sockets. The supported ends of the measuring bars or struts may therefore be provided with correspondingly shaped balls or sockets at either end as appropriate, which may be completely spherical or only part-spherical, in order to fit on the supports.
The measuring bars or struts may extend between the supports and be telescopic or extensible in some other way, or may be of fixed length and extend through, or carry extensions which extend through, the supports so as to provide a greater range of relative movement for the structures.
The struts may be powered by internal or external drive means as known per se.
According to one novel feature of the invention, the supported end of a measuring bar or strut lies at an angle to the longitudinal axis of the strut, whereby, when in position on the support, the axis of the strut is aligned with the centre of the support.
The measuring bars or struts may be attached to the supports in any convenient manner, but a preferred form of attachment is by means of magnetic attraction.